In the working of workpieces with a chip-removing rotating tool, the feed movement of the tool must stop as closely as possible to a point spaced a predetermined small distance between the active tool surface and the workpiece surface, so that in consideration of this predetermined distance the final feed of the tool can progress at the desired amount. Furthermore, it is also desired that the feed movement which takes place as much as possible in order to save time in a rapid and power driven traverse, upon reaching a selected distance is switched over from rapid and power driven traverse to creep feed and this feed movement is then ended upon reaching a selected still smaller distance between the active tool surface and the workpiece surface. A device for ending the feed movement is particularly useful in the case of grinding wheels, because these wear as is known and must then be dressed. After dressing they have a smaller diameter, whereby the correction of this diameter difference is time consuming upon renewal of feed after the dressing if one is to be successful in limiting the advance of the grinding wheel feed so that the active surface of the grinding wheel assumes a predetermined distance from the workpiece surface.
Mechanically operating stopping devices are known, in which the feed path is detected through a feeler system contacting the workpiece. For this purpose, there exists also a device in which the feeler system is connected with the sleeve of a dressing device, so that the reduction of the grinding wheel diameter which is caused by the dressing operation is compensated (German OS No. 2,135,473). However, this mechanical system is susceptible to wear, namely clearance in its moving parts and also to chips or dust from the workpiece. Starting or positioning errors of the feeler on the workpiece surface can also cause inaccuracies.
A different known stopping device (German Patent No. 1,107,553) uses a thin feeler band of metal, plastic, wax paper and the like, the one end of which is connected through a clamping device to a switch acting on the infeed. The feeler band is placed onto the workpiece to be worked. Upon the approach of the grinding wheel, the feeler band is pulled between grinding wheel and workpiece and acts through the electric switch of the feed drive to stop the grinder. The thickness of the feeler band determines the distance between the active tool surface and the workpiece surface. However, operation of this portable device is complicated, because it must always be brought to the grinding point, whereupon the necessary connection of a cable through the device to the control panel or the control cabinet of the machine makes handling more difficult. Furthermore, the feeler band must be held manually between workpiece and grinding wheel, which particularly in the case of profile grinding can present difficulties. Finally it is necessary after each infeed to clamp a new piece of feeler band into the clamping device, because this feeler band is torn off upon contact of the grinding wheel.
Also stopping devices exist in which the feed drive is turned off, as soon as the grinding wheel has contacted the workpiece to be worked (German OS Nos. 2,044,083, 2,045,314 and spark indicator 9176 of the Firm CEJ Forster, Bad Nauheim). However, in these devices a disadvantageous requirement is that the grinding wheel must already be in engagement with the workpiece in order to deliver information controlling the infeed. If one considers the slowing-down time required in feed drives due to the mass forces, it will be understood that only limited fields of application are available for these devices, such as for rough grinding.